At this stage, there is no way you will be able to get the same runtime as you did with the original firmware.
The port is very young and there is a lot of fine tuning left to do, Rockbox makes no claims about the usability of this player (in fact, it is specifically labeled as 'unusable'). You are most certainly welcome to learn some C (and possibly some ARM ASM), set up a development environment, check out a copy of the Rockbox git repository, and see if you are able to make some contributions towards the further progress of the port if you would like to see the port status and usability advance in the future.
[Saint]